1. Field of the invention
The invention relates to a vehicle deceleration force control device and control method thereof for adding a deceleration force to a vehicle, and more particularly to a vehicle deceleration force control device and control method thereof for adding deceleration force to a vehicle in accordance with an operation state of an accelerator.
2. Description of the Related Art
There is known a braking force control device that adds deceleration force to a vehicle by actuating a braking force upon release of a depressed accelerator pedal by a driver. Japanese Patent Application Laid-Open Publication No. 9-95222 discloses a deceleration force control device that adds braking force to a main brake system through the braking force control device when the accelerator pedal is in a deceleration range.
In a vehicle providing such a braking force control device, a brake pedal does not need to be operated frequently when gently accelerating or decelerating the vehicle and the deceleration responsiveness is enhanced. Accordingly, driving is made easier.
However, according to the above-mentioned braking force control device, there is a problem that driveability deteriorates when the accelerator pedal is depressed from a deceleration range to an acceleration range, due to a rapid decrease of braking force to the vehicle, which leads to discontinuous deceleration and acceleration from a point where the vehicle""s deceleration stops up to a point where and the vehicle starts accelerating.
Accordingly, in view of the above-mentioned problem, it is an object of the invention to provide a vehicle deceleration force control device and control method thereof that enables smooth acceleration when shifting from deceleration to acceleration.
To solve the above problem, a vehicle deceleration force control device, which is a first aspect of the invention, includes an accelerator operation state detector which detects the accelerator operation state of a driver, a deceleration force adding device which adds deceleration force to the vehicle, and a control portion which adds the deceleration force by controlling the deceleration force adding device when it is determined from the detection results of the acceleration operation state detector that the accelerator operation state is in a return operation state. The control portion implements a deceleration force limiting control for reducing the deceleration force to be added by the deceleration force adding device over a time equal to or greater than a predetermined time when it is determined that the accelerator operation state has changed to an acceleration operation state when deceleration force is being added to the vehicle by controlling the deceleration force adding device.
According to the invention, the vehicle can be smoothly decelerated by actively adding deceleration force thereto when the accelerator operation state is in a return operation state. When a driver operates the accelerator to shift from a deceleration state to an acceleration state, the deceleration force is reduced over a predetermined time or longer. Accordingly, an acceleration shock generated by a rapid decrease of deceleration force before a driving force of an engine actually increases and achieves acceleration is controlled, thereby enabling natural and smooth acceleration.
It is preferred that the control portion implement this deceleration force limiting control by controlling the change amount of deceleration force over time that is added by the deceleration force adding device at a predetermined value or less. Accordingly, a shift from a deceleration state to an acceleration state can be made smoothly.
In the first aspect, a gradient determination device for determining a road gradient may further be provided. It is preferred that the control portion set a predetermined value for limiting the change amount over time according to the determination results of the gradient determination device. Further, it is preferred that the predetermined value be set to a larger value the steeper the road gradient during hill climbing, and to a smaller value the steeper the road gradient during hill descent. During hill climbing, since deceleration force is added by gravitational force the steeper the road gradient, the vehicle speed decreases even if deceleration force is not actively added. Accordingly, even when the added deceleration force is rapidly decreased, the decrease in the actual vehicle speed is small. That is, the acceleration shock to the driver as a consequence of rapid decrease of deceleration force is small. Also, since more acceleration force is added by gravitational force the steeper the road gradient during hill descent, it is necessary to gradually reduce the deceleration force.
It is preferred that this deceleration force adding device add deceleration force by controlling the braking force. In the case where a mechanism which assists the braking force with respect to the brake operation force is adopted, the invention can be realized with a simple modification.
A vehicle deceleration force control method, which is another aspect of the invention, detects an accelerator operation state of a driver, determines whether or not the accelerator operation state is in a return operation state based on the detected acceleration operation state, adds a deceleration force to the vehicle when it is determined that the acceleration operation state is in the return operation state, determines whether or not the accelerator operation state has changed to an acceleration operation state while the deceleration force is being added to the vehicle, and reduces the deceleration force added to the vehicle over a time equal to or greater than a predetermined time when it is determined that the accelerator operation state has changed to the accelerator operation state.
According to this aspect, the rapid decrease of deceleration force is controlled when the deceleration state is changed to an acceleration state by accelerator operation during engine brake assist control, leading to a smooth shift from deceleration to acceleration, thereby improving driveability.